Plastic films can be continuously produced as films with large areas, though this cannot be achieved with other materials. Because of their features of high strength, durability, clarity, flexibility and good surface properties, they are used in various fields needing them in large quantities such as magnetic recording media, agriculture, packaging, building materials, etc.
Above all, biaxially oriented polyester films are used in various fields since they are excellent in mechanical properties, thermal properties, electric properties and chemicals resistance, and especially as base films for magnetic tapes, they are incomparable to any other materials.
In this field, especially in recent years, the base films are further requested to be thinner for meeting the demands for lighter-weight and smaller-sized devices and longer-time recording capability. The films for heat transfer ribbons, capacitors and thermal mimeographic stencil paper are also highly requested to be thinner in recent years.
However, if a thinner film is produced, the mechanical strength becomes insufficient to make the film less stiff and likely to be elongated. So, for example, in the application as a magnetic recording medium, the tape is likely to be damaged, or the head touch becomes poor to lower electromagnetic conversion properties. Furthermore, if a thinner film is used as a heat transfer ribbon, the ribbon cannot be kept flat during printing, to cause irregular printing or over-transfer. A thinner film used as a capacitor lowers the dielectric breakdown voltage disadvantageously.
In the demand for thinner films, films are desired to have higher strength, by improving mechanical properties such as tensile properties including Young's modulus.
So, various methods have been studied to enhance the strengths of films.
A generally known method for enhancing the strength of a biaxially oriented polyester film is to re-stretch a once longitudinally and laterally stretched film in the longitudinal direction for enhancing the strength in the machine direction as the so-called longitudinal re-stretching method (e.g., Japanese Patent Publication (Kokoku) Nos. 42-9270 and 43-3040, Japanese Patent Laid-Open (Kokai) Nos. 46-1119 and 46-1120, etc.) For further enhancing the strength also in the transverse direction, it is proposed to re-stretch said longitudinally re-stretched film as the longitudinal re-stretching and lateral re-stretching method (e.g., Japanese Patent Laid-Open (Kokai) Nos. 50-133276 and 55-22915, etc.). Furthermore, it is proposed to once stretch the film in the longitudinal direction in 2 or more steps and then to stretch in the lateral direction as the multi-step longitudinal stretching method (e.g., Japanese Patent Publication (Kokoku) Nos. 52-33666 and 57-49377, etc.).
The multi-step longitudinal stretching method is superior to the longitudinal re-stretching method and the longitudinal re-stretching and lateral re-stretching method in view of higher strength, less film thickness fluctuation and higher productivity. However, the problem that a film with a higher strength becomes larger also in heat shrinkage and is more frequently broken cannot be solved by the multi-step longitudinal stretching method either.
It is also proposed to stretch a film three or more times continuously repetitively in at least either the machine direction or the transverse direction as the small-ratio repetitive stretching method (supermulti-step stretching method) which is one of conventionally known film production methods and is similar to the method of the present invention described later (Japanese Patent Laid-Open (Kokai) Nos. 8-224777 and 9-57845). However, the inventions described in said Japanese Patent Laid-Open (Kokai) Nos. 8-224777 and 9-57845 simply show examples of mainly sequential biaxial stretching, and do not refer specifically to any mechanism, apparatus or process conditions effective for simultaneous biaxial stretching. In addition, they do not refer to the effectiveness of the simultaneous biaxial stretching method by a linear motor system proposed to be used as a preferable apparatus in the present invention since it allows high ratio stretching.
On the other hand, in recent years, linear motor driven simultaneously biaxially stretching tenter ovens have been developed, and attract attention because of their high film forming speeds, etc. (e.g., Japanese Patent Publication (Kokoku) No. 51-33590, U.S. Pat. Nos. 4,853,602 and 4,675,582, etc.).
The conventional simultaneous biaxial stretching methods such as the screw method for spreading the clip interval by guiding clips in the grooves of screws and the pantograph method for spreading the clip interval using a pantograph have such problems that the film forming speed is low, that it is not easy to change conditions such as stretching ratio, and that stretching at a high ratio is not easy. On the contrary, the linear motor driven simultaneous biaxial stretching method has possibility to solve these problems all at once.
Said Japanese Patent Publication No. 51-33590 discloses to change the tenter clip interval by the electric force generated by a linear motor for allowing highly efficient production. Furthermore, said U.S. Pat. No. 4,853,602 discloses a stretching system using a linear motor, and said U.S. Pat. No. 4,675,582 discloses a system effective for controlling many linear motors along the stretching section. However, even these U.S. patents do not refer to the stretching method disclosed in the present invention or the high quality polyester film intended to be obtained by said method.
The process conditions for producing a polyester film with excellent film properties and quality by the linear motor driven simultaneous biaxial stretching were unknown, and any effective stretching method was not established yet.
As described above, the techniques for producing a polyester film with high film properties and quality are yet to be improved, and it has been demanded to develop a new technique in this industrial field.